Many sexual assault victims experience an involuntary inability to move during rape, and those who do are more likely to later develop posttraumatic stress disorder (PTSD) and depression, Swedish researchers say.

In a large sample of women treated at a rape clinic in Stockholm, about 70 percent reported an inability to move or resist during the assault.

“Victims tend to blame themselves for not resisting, so the fact that 7 out of 10 have paralysis is useful to know,” said lead author Dr. Anna Moller of Stockholm South General Hospital and the Karolinska Institute.

In animals, this reaction, known as tonic immobility, is an evolutionary defensive response to a predatory attack, Moller’s team wrote June 7 online in Acta Obstetricia et Gynecologica Scandinavica.

Humans can similarly feel paralyzed when under attack or in other life-threatening circumstances, experiencing temporary, involuntary immobility that may also involve a dissociated, catatonic-like mental state.

“In court situations, a lot of focus is given to whether the victim consented to the intercourse or not,” Moller told Reuters Health by email. “This paralysis and passiveness should not be considered consent.”

Moller and colleagues surveyed 298 women who visited the Emergency Clinic for Raped Women in Stockholm within one month of a sexual assault. The clinic offers free services and sees about 600 patients per year. The researchers interviewed participants using a questionnaire called the Tonic Immobility Scale, which asks about feeling frozen or paralyzed, the inability to move although not restrained, the inability to call out or shout, numbness, feeling cold and feeling detached from one’s self.

After six months, the research team also assessed 189 of those women for PTSD and depression.

They found that 70 percent of women reported significant tonic immobility and 48 percent reported extreme tonic immobility during their assaults. Having had this experience more than doubled the likelihood of PTSD at the six-month follow up and raised the likelihood of severe depression by nearly four-fold.

Of the 189 women who completed the six-month assessment, more than half of those who experienced tonic immobility during their assault had developed PTSD, compared to 28 percent who didn’t. Similarly, 34 percent of those who experienced tonic immobility were severely depressed, compared to 14 percent of those who didn’t.

The research team also found that women with a history of sexual assault – in childhood or adulthood – were twice as likely to have experienced tonic immobility during the recent rape. Women whose rape was part of a “more severe assault,” involving physical violence or multiple attackers, for example, were also twice as likely to have experienced tonic immobility during the attack.

One limitation of the study is that the emergency clinic only saw female patients at the time of the interviews. Moller’s team would also like to include larger, more diverse samples in several geographic locations to better understand tonic immobility generally, she said.

Future studies should also include levels of victim resistance, said Jennifer Wong of Simon Fraser University in Vancouver, who wasn’t involved in the research.

“A victim may have felt frozen, detached or numb during some of the assault but may have initially verbally or physically resisted her attacker,” Wong said.

Tonic immobility research could have major implications for court cases and the psycho-education of rape victims, Moller said.

“Don’t blame rape victims for not resisting,” she said. “It’s a normal biological reaction when exposed to an extreme threat.”